The central hypothesis of this proposal is that estrogen induces the expression of brain derived neurotrophic factor (BDNF) in hippocampus, and this leads to changes in neuronal excitability that ultimately increase the susceptibility to seizures. This hypothesis is relevant to women with epilepsy, such as those with catamenial epilepsy, or those taking estrogen. It may also allow greater insight into seizures that involve the hippocampus. The proposed studies are based on preliminary data which indicate that the rise in estrogen during the normal female rat estrous cycle is accompanied by increased BDNF expression in hippocampal dentate granule cell axons, the mossy fibers. Thus, mossy fiber BDNF appears greater in animals killed on the morning of proestrus, when estrogen is elevated, than animals killed on the morning of metestrus, when estrogen is low. Hippocampal slices made at proestrus demonstrate increased excitability in area CA3, the major target of mossy fibers, but only when the mossy fiber pathway is stimulated. In addition, other experiments indicate that there is increased susceptibility to the convulsant pilocarpine at proestrus relative to metestrus. We will first test the hypothesis raised by these data, that the rise in estrogen that occurs at proestrus is accompanied by increased mossy fiber BDNF and physiological changes in mossy fiber transmission. Therefore, we will examine BDNF expression and mossy fiber transmission at different stages of the rat estrous cycle. In addition, awake animals with implanted hippocampal electrodes will be used to examine possible electrographic changes coincident with changes in estrogen. Awake animals will also be used to compare afterdischarge threshold and convulsant sensitivity at different cycle stages. Complementary experiments will be conducted using ovariectomized rats, with and without hormone replacement. To address the mechanism of hyperexcitability at proestrus, we will test the hypothesis that elevated BDNF at proestrus enhances mossy fiber transmission by an action at alpha7 nicotinic cholinergic receptors (alpha7nAchRs) on mossy fiber boutons. This hypothesis is based on preliminary data demonstrating that a high affinity neurotrophin receptor (trk) antagonist or a selective alpha7nAChR antagonist can normalize hyperexcitability at proestrus. Taken together with previous studies showing that BDNF induces alpha7nAChRs to cluster (Kawei et al., 2002), and evidence that alpha7nAChRs are present on mossy fibers boutons (Gray et al., !996), we hypothesize that BDNF "primes" the mossy fibers to increase glutamate release by influencing alpha7nAchRs. In summary, these studies will provide a comprehensive and mechanistic analysis of a neuroendocrinological hypothesis that has relevance for hippocampal function and dysfunction.